Timeline: The scientific history of Pluto

1906: American businessman, mathematician, and astronomer Percival Lawrence Lowell spearheaded an extensive project to search for a possible ninth planet that he called Planet X. His team used the Lowell Observatory in Flagstaff, California.

1909: Lowell and American astronomer William Henry Pickering had provided several possible celestial coordinates for Planet X. The entire team continued the search until the death of Lowell in 1916. However, unknown to Lowell, his surveys had captured two faint images of Planet X. The first two images were dated on 19 March 1915 and 17 April 1915 respectively.

It is also worth mentioning that there are 14 other known prediscovery observations. The oldest was made on 20 August 1909 by the Yerkes Observatory of the University of Chicago.

1929: Following the death of Lowell, his widow, Constance Lowell, embarked on a 10-year legal battle and attempted to seize the million-dollar portion of Lowell Observatory for herself. Thus, the search for Planet X was halted until 1929.

Vesto Melvin Slipher, director at Lowell Observatory, handed over the job of finding Planet X to Clyde Tombaugh, a 23-year-old astronomer from Kansan who impressed the observatory director with his astronomical drawings.

1930: Specifically on 18 February 1930, after systematically imaging the night sky in pairs of photographs to determine any objects that had shifted position, Tombaugh discovered such object and movement on photographic plates captured on January 23 and 29. Another lesser-quality photo taken on January 21 further confirmed the object and movement.

On March 13, after Lowell Observatory took additional confirmatory photographs, news of the discovery was telegraphed to the Harvard College Observatory.

The discovery captured the attention of the media and the public across the globe. Because Lowell Observatory made the discovery, it had the right to name the object. The observatory received thousands of suggestions but only three of them had gone through the final selection phase. These are Minerva, Cronus, and Pluto.

Each member of Lowell Observatory had the chance to vote. Pluto received a unanimous vote and it became the official name of the object starting March 24. The name was announced on May 1.

As a backgrounder, the name Pluto was inspired from the god of the underworld of Greek mythology. Then 11-year-old schoolgirl from Oxford, England Venetia Burney proposed the name during a conversation with her grandfather, Falcon Madan, a former librarian at the University of Oxford. Madan passed the suggestion to astronomy professor Herbert Hall Turner who, on the other hand, cabled the name to his colleagues in the United States.

1931-1976: The faintness of Pluto and its lack of resolvable disc created doubts whether it was in fact the Planet X of Lowell. Estimates of its mass were revised downward. For instance, in 1915, Lowell predicted that the ninth planet would be seven times larger than Earth. In 1931, Nicholson and Mayall estimated that Pluto and Earth were similar in mass.

Kuiper estimated in 1948 that Pluto was 1/10 of Earth while Cruikshank, Pilcher, and Morrison estimated it to be 1/100 of Earth.

1978: The Plutonian moon Charon was discovered. This discovery allowed the first-ever exact measurement of Pluto—roughly 0.2 percent or 1/500 of Earth. This was far too small to account for the discrepancies in the orbit of Uranus.

1992: Several objects were discovered starting from 1992 and onwards that orbit in the same area as Pluto. These discoveries suggested that Pluto might be just a part of a population of objects called the Kuiper Belt.

It is important to note that since the discovery of Pluto in 1930, many speculated that the object might not be alone in the nearby area. These objects were popularly called trans-Neptunian objects or TNO. By definition, a TNO is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune. The speculations nonetheless formed the first hypothesis that predated the Kuiper Belt. However, in 1992, the first direct evidence of the belt was found.

The discovery of the Kuiper Belt made Pluto controversial. Others suggested that Pluto should be considered as an object that formed part of a greater population of trans-Neptunian objects. This would reclassify Pluto as a minor planet. However, some maintained that the object should retain its status as a planet.

2005: A new trans-Neptunian object called Eris was discovered. The object is currently the most massive and second-largest minor planet in the Solar System. Other members of the scientific community argued that this discovery is the strongest evidence for reclassifying Pluto as a minor planet.

2006: The International Astronomical Union released a resolution on 24 August 2006 to resolve the issues and debate surrounding Pluto and the classification of other objects within the Solar System. Thus, they came up with a new, official, and universal definition for the term planet.

Accordingly, in order for an object to be considered as a planet, it should meet the following conditions: (1) The object must be in orbit around the sun; (2) The object must be massive enough to be rounded by its own gravity—specifically, its gravity should pull it into a shape of hydrostatic equilibrium; (3) It must have cleared the neighbourhood around its orbit.

Pluto fails to meet the third criterion. This prompted IAU to reclassify the object as a dwarf planet on 13 September 2006. The scientific community has since then remained divided about the new classification of Pluto.

The American Dialect Society voted “plutoed” as the word of the year in 2006. To “pluto” is to demote or devalue someone or something.

Earlier in 2006, specifically on January 19, the National Aeronautics and Space Administration or NASA launched the New Horizons mission. This is the first reconnaissance mission aimed at exploring the edge of the Solar System, particularly by exploring Pluto and the Kuiper Belt.

2008: From 14 to 16 of August 2008, researchers on both sides of the debate gathered at the John Hopkins University Applied Physics Laboratory for a conferenced called “The Great Planet Debate” aimed at discussing the current IAU definition of a planet. The IAU published a post-conference press release mentioning that the community remains unable to come up with a consensus about the definition of planet.

2015: On 14 July 2014, the New Horizons mission of NASA, particularly the New Horizons spacecraft flew through the Pluto system— the first-ever flyby of the faraway dwarf planet, zooming within 7,800 miles or 12,500 kilometres of its frigid surface. Telemetry data confirming a successful flyby and a healthy spacecraft were received on Earth from the vicinity of the Pluto system on 15 July 2015.